1. Field of the Invention
This invention relates to a method of detecting the sensitivities of photomultipliers.
2. Description of the Prior Art
Photomultipliers are suitable for detecting very weak light and are widely used as a light detector, for example, in image read-out systems wherein an image recorded on a sheet is read out by two-dimensionally scanning the sheet with a light beam such as a laser beam, and detecting the light (for example, reflected light or transmitted light) carrying the image information obtained by the scanning.
Such an image read-out method using photomultipliers as described above is employed, for example, in a radiation image recording and reproducing system using a stimulable phosphor sheet. The radiation image recording and reproducing system is disclosed, for example, in U.S. Pat. No. 4,258,264 and Japanese Unexamined Patent Publication No. 56(1981)-11395. In this system, a stimulable phosphor sheet carrying a radiation image stored therein is exposed to stimulating rays which cause the stimulable phosphor sheet to emit light in the pattern of the stored image, and the emitted light is detected by use of photomultipliers.
The term "stimulable phosphor" as used herein means such a phosphor that when exposed to a radiation such as X-rays, .alpha.-rays, .beta.-rays, .gamma.-rays, or ultraviolet rays, or the like, it absorbs and stores a part of the energy of the radiation and emits light in proportion to the stored energy when excited with stimulating rays such as visible light and infrared rays after exposure to the radiation.
In the aforesaid radiation image recording and reproducing system using a stimulable phosphor, image read-out is conducted as described below. Namely, a stimulable phosphor sheet is first exposed to a radiation such as X-rays passing through an object such as a human body to have a radiation image stored therein, and is then two-dimensionally scanned with stimulating rays such as a laser beam which cause the stimulable phosphor sheet to emit light in the pattern of the stored image. The light emitted from the stimulable phosphor sheet upon stimulation thereof is photoelectrically detected by use of photomultipliers and converted to an electric image signal, which is processed as desired to reproduce a visible image on a recording medium such as a photographic light-sensitive material or on a display device such as a cathode ray tube (CRT).
In the radiation image recording and reproducing system as described above, particularly in the radiation image recording and reproducing system using a plurality of photomultipliers, in order to assure normal operation of the system, it is necessary that at least the sensitivities of the respective photomultipliers relative to one another always be set to a predetermined value.
However, the sensitivity of a photomultiplier changes (deteriorates) with the lapse of time. Also, it changes according to the change of ambient temperature. When the sensitivity is changed, it becomes impossible to read out the light carrying the image information accurately. Particularly in the system using a plurality of photomultipliers, when the sensitivities of the respective photomultipliers relative to one another deviates from a predetermined value, the system cannot work normally. A change in the sensitivities of many photomultipliers relative to one another presents a very real problem particularly in the aforesaid radiation image recording and reproducing system using a stimulable phosphor sheet.
It has been proposed in U.S. patent application Ser. No. 434,886, now U.S. Pat. No. 4,527,060 (European patent application Ser. No. 82305530.6) to conduct, in the radiation image recording and reproducing system, a read-out operation for detecting the image input information of a radiation image stored in a stimulable phosphor by use of stimulating rays having stimulation energy lower than the stimulation energy of stimulating rays used in a read-out operation for obtaining a visible image for viewing, particularly for diagnostic purposes, and thereafter to conduct the read-out operation for obtaining a visible image for viewing, that is, the final read-out. The above-mentioned read-out operation conducted before the final read-out operation is hereinbelow referred as "preliminary read-out".
In the final read-out, the read-out gain is adjusted, and/or the scale factor is determined, and/or the image processing conditions are determined appropriately based on the image input information obtained by the preliminary read-out, thereby to obtain a radiation image free from fluctuations in the radiographic exposure conditions and suitable for viewing, particularly for diagnostic purposes. The read-out gain and the scale factor are together referred to as the read-out conditions. In this case, the image input information obtained by the preliminary read-out must be sufficiently accurate to reliably determine the read-out conditions in the final read-out and/or the image processing conditions. To accomplish this, it is necessary that the sensitivity of the photomultiplier used in the preliminary read-out and the sensitivity of the photomultiplier used in the final read-out be strictly adjusted at least relative to each other.
Further, it has been proposed in Japanese patent application No. 56(1981)-179287 to measure the level of the radiation energy remaining in a stimulable phosphor sheet after a radiation image stored in the stimulable phosphor is read out, and to control the amount of light irradiated to the stimulable phosphor sheet for erasing noise according to the measured level of the residual radiation energy. In this case, noise erasing cannot be conducted securely if the sensitivity of the photomultiplier changes.
As described above, in a system provided with a plurality of photomultipliers, if the sensitivities of the respective photomultipliers relative to one another deteriorate with time or change due to a change in the ambient temperature, the whole system is prevented from working normally. Therefore, it is necessary to always maintain the sensitivities of the respective photomultipliers relative to one another at a predetermined value.